1. Field of the Invention
The present invention relates to ink-jet recording, particularly to an ink-jet recording method capable of obtaining a high quality image on a recording medium mainly comprising plain paper, and to an ink-jet ink (abbreviated as “ink” hereinafter) used thereto. The invention further relates to an ink-jet recording unit, ink cartridge for ink-jet recording and ink-jet recording apparatus.
2. Description of the Related Art
The ink-jet recording method is the method of ejecting small droplets of ink by applying energy, and recording by landing droplets on a recording medium such as paper. Particularly, in the method using an electricity-heat conversion member as an ejection energy supply device for ejecting the droplets by allowing bubbles to be generated by applying heat energy to the ink, a recording head is readily made to have high density nozzles to enable high resolution quality images to be recorded in a high speed (see Japanese Patent Publication Nos. 61-59911, 61-59912 and 61-59914).
However, since the conventional ink used for ink-jet recording usually comprises water as a principal component with supplemented water-soluble high boiling point solvents, such as glycols for preventing drying and clogging, the ink permeates deep into the recording paper when images are recorded on plain paper using such ink. Consequently, sufficient image densities cannot be obtained, and the image densities become uneven probably due to uneven distribution of fillers and sizing agents on the surface of recording paper. Since a plurality of color inks are printed one after another before the precedent inks are fixed when a color image is printed, a satisfactory image cannot be obtained due to blur of colors and uneven mixing (referred to as “bleeding” hereinafter) at the boundaries of different colors.
For solving the above problems, an ink having a surface tension less than or equal to the critical surface tension of the recording medium is used, for example, by adding compounds capable of enhancing permeability of the ink, such as glycol ethers and surfactants (Japanese Patent Application Publication Nos. 60-34992 and 62-28828). However, although permeability of the ink into recording paper (recording medium) is improved suppress bleeding to a certain extent, the image density is decreased and clearness of the image is deteriorated because the ink permeates deep into recording paper together with colorants. This method is not preferable because the ink tends to be spread due to improved wettability of the surface of recording paper causing a decrease of resolution and blur. This problem is evident when black letters are printed.
On the other hand, when the surface tension of the ink is adjusted to be higher than the critical surface tension of the recording medium, fixing speed is impaired and bleeding performance becomes quite poor, although the problems of decreased image density and clearness and deteriorated quality of the black letters do not occur.